Constipated toddler

My child was complaining of back pain (she’s almost 3) and I took her to the doctor she got an X-ray that showed a moderate amount of stool. They prescribed miralax but I’ve read that it is not FDA approved for children and has side effects. I wanna try the natural way before I give my kid this stuff everyday for a month. I myself have tried miralax because I’ve had issues with constipated since I was in highschool. It did not work at all for me. Just made my stomach upset. Any recommendations and foods I can give her that she will actually eat? She’s so picky. She never wants juice only water. I try to give her fruits and the only one she’ll eat sometimes is peaches in the cup.